In the figure given $\tan a = x$. What is the value of $\cos b$ ?

Question

$\begin{array}{1 1} (a)\;\dfrac{1}{\sqrt{1+x^2}} \\ (b)\; \dfrac{1}{\sqrt{1-x^2}} \\ (c)\; \dfrac{x}{\sqrt{1+x^2}} \\ (d)\; \dfrac{x}{\sqrt{1-x^2} }\end{array}$